The overall objective of this project is to elucidate the mechanisms by which the nuclear receptors nor-1 and nur77 function as tumor suppressors to prevent the development of myeloid leukemia. Nur77 and nor-1 are members of the NR4A subfamily of nuclear receptor transcription factors. NR4A receptors have highly homologous amino acid sequences and can interact with common cis-acting DNA elements to regulate overlapping target genes. Unlike most nuclear receptors, NR4A subfamily members are not ligand activated and can function as constitutively active transcription factors. In addition, they are products of immediate early genes whose expression and activity are regulated in a cell specific manner in response to a variety of extracellular mitogenic, apoptotic and differentiative stimuli. In order to elucidate the essential physiological roles of NR4A receptors, we have generated null mutant mice in which the expression of these proteins has been ablated. The current proposal is based on our recent discovery that mice deficient in both nur77 and nor-1 develop rapidly lethal myeloid leukemia that is nor1/nur77 gene dosage dependent in its latency of development and most closely resembles the acute phase of chronic myeloid leukemia (CML). We hypothesize that norl and nur77 are essential tumor suppressor transcription factors operating most likely in hematopoietic stem (HSCs) or downstream myeloid progenitor cells and exert their effects by communication with cell cycle components to regulate the balance between serf renewal, proliferation and cell survival. We further hypothesize that inactivation of nor1/nur77 may play a key role in driving progression of chronic myeloproliferative disease to acute phase. To test this hypothesis, we will 1) continue to examine the cellular mechanisms of initiation and progression of myeloid leukemia in nor1/nur77 null mutant mice, 2) identify the nor1/nur77 dependent molecular genetic signaling pathways that control myelopoietic cell development, 3) determine whether loss of nor1/nur77 cooperates with BCR-ABL oncogenic signaling to drive acute phase progession in a mouse model of CML, and 4) determine whether transgenic targeting of norl and nur77 to myeloid progenitor cells is sufficient to rescue the leukemic phenotype in nor1/nur77 null mice.